I changed the fonts of my N900 on another way than overwrite the four Nokia fonts.
I placed my own font in /usr/share/fonts/nokia and changed it in the
gtkrc file (/usr/share/themes/*THEME-NAME*/gtk-2.0)
With this method I can decide wich font is changed. (See screenshot: white letters changend but the orange smaller are orginal font)

Part from gtkrc:

##############################
#
# Font definitions
#
##############################

# legacy

style "osso-SystemFont" {
font_name = "Nokia Sans 18"
}

....

###########################
# new fonts for fremantle #
###########################

style "SystemFont" {
font_name = "Nokia Sans 18"
}

style "EmpSystemFont" {
font_name = "Nokia Sans Bold 18"
}

style "LargeSystemFont" {
font_name = "Nokia Sans 26"
}

style "SmallSystemFont" {
font_name = "Nokia Sans 13"
}

style "EmpSmallSystemFont" {
font_name = "Nokia Sans Bold 13"
}

style "X-LargeSystemFont" {
font_name = "Nokia Sans 31"
}
....